It is pointed out that an airbag of a vehicle has a potential for disturbing smooth breathing when a passenger is, for example, a child at the time of inflating the airbag. It is also pointed out that in the case of a child seat being installed on a front passenger's seat in a rear-facing state, there is a potential to push the child seat backwards to give a shock to a child who is seated on the child seat, when the airbag is inflated. In order to solve the above problem, there has been a well-known technique to determine the weight of a seated person to distinguish between an adult and a child or between a man of big physique and a man of small physique so as to perform inflation control of the airbag, attitude control of the seat, or the like.
For instance, Patent Document 1 (Japanese Patent Publication Laid-open No. 2003-25890) discloses a configuration of installing a device for determining a load on a seat composed of a combination of a sensor block and a strain gauge on a lower portion of a guide rail supporting the seat.
However, as disclosed in Patent Document 1, since the conventional device for determining a load on the seat is installed on the lower portion of the guide rail, in order to provide the device for determining the load, it is necessary to change the design of the entire seat structure such as adjustment of an installation position of a slide adjuster of the seat and so on. This is because an ordinary seat used in a vehicle or the like is structured such that a cushioning member made of polyurethane foam having a thickness of about 10 cm is mounted on a seat frame, which forces something to detect the load of the seat to dispose beneath the seat.
On the other hand, the present applicant proposes a seat structure in which the rear of the cushioning member of the seat cushion is allowed to be elastically supported via a torsion bar so that the tension of the cushioning member can be made different between the times when someone is seated and no one is seated, and improvement in vibration absorbing characteristic, impact absorbing characteristic, body pressure dispersibility, attitude supportability, and so on can be achieved. In such a seat structure, since the rear portion of the cushioning member is supported by a supporting frame connected to the torsion bar disposed on the rear of a seat, when a person takes the seat, the torsion bar is twisted so that the supporting frame is pivoted at an angle to tilt forwards. Accordingly, when a displacement detecting mechanism to detect a twist of the torsion bar or the rotation angle of the supporting frame is arranged on such a seat structure, determination of the load on the seat becomes possible without changing the basic design of a seat structure required to exhibit the above-described vibration absorbing characteristic and impact absorbing characteristic, and without affecting the design of attachment mechanisms such as a slide adjuster, a reclining mechanism and so on.